As soon as she sees the first few tears drip; as soon as she hears the first soft sobs, Elsa's shuffling forwards to be closer. It's an awkward angle and yet she still succeeds in pulling Anna close and wrapping her arms around her.
Anna's reaction only reminds her of just how young Anna is; was it unfair of her to say anything at all? Undoubtedly, Anna would have found happiness, and with someone entirely more appropriate than Elsa.
But then Anna looks at her, and hugs back just as tight, and Elsa can't regret anything.